I'd say it's the brakes. They are so strong and effortless, letting you rip with confidence. When you switch to another bike it quickly becomes apparent how great the Busa's brakes are.
Close second the cruise control, it is a huge quality of life improvement on the highway and crossing small villages. The speed limiter is nice too, I don't use it much, but I'm sure it has saved me a couple hundred Euros in mountain passes with strictly enforced speed limit.